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The present invention relates to a device for the medical treatment of and/or operation on the eye with laser radiation making use of auxiliary radiation for determining the eye position.
Such a device can especially be used for the so-called PRK (photorefractive keratectomy), i.e. a method of correcting a defective vision of the human eye in the case of which especially the cornea is re-shaped. The device according to the present invention is particularly suitable for the so-called LASIK method where a small flap (lid) is first cut into the epithelium, Bowman""s membrane and stroma and opened up, the PRK being then carried out in the stroma.
In the laser treatment of the human eye, the use of various lasers is known; in connection with the present invention these lasers are in particular excimer lasers (having a wavelength of e.g. 193 nm) and also Er:YAG solid state lasers.
In the case of PRK (especially LASIK) material of the cornea is ablated. The ablation is a function of the energy density (energy per unit area) of the laser beam impinging on the cornea. Different techniques for forming and controlling the beam are known, e.g. the so-called slit scanning in the case of which the radiation is guided over the area to be treated by means of a moving slit, the so-called spot scanning in the case of which a radiation spot having very small dimensions is guided over the area to be ablated, and also the so-called full ablation in the case of which large-area application of the radiation is effected over the whole region to be ablated and the radiation density varies across the beam profile so as to achieve the desired ablation of the cornea. Suitable radiation-control algorithms for the respective methods of controlling the beam are known in the prior art so as to ablate the corneal surface such that the cornea will finally be given the desired radius of curvature.
Spot scanning, which has already been mentioned hereinbefore, uses a laser beam focussed to a comparatively small diameter (0.1-2 mm), said laser beam being directed onto various points of the cornea by means of a beam control unit and successively moved by means of a so-called scanner in such a way that the desired corneal ablation will finally be achieved. For the purpose of PRK, so-called galvanometric scanners can especially be used (cf. the article, G. F. Marshall in LASER FOCUS WORLD, Jun. 1994, p. 57). The present invention refers in particular to the so-called spot scanning executed in the LASIK method.
A special problem arising in connection with PRK and LASIK is the relative positioning of the laser beam and of the eye. A mechanical fixation of the eye is not satisfactory for medical reasons. Hence, the prior art knows a so-called optical fixation in the case of which a so-called fixation beam is used, which is normally coaxial with the material-ablating laser beam. The patient is requested to look precisely at the point defined by the fixation beam so that the eye will always maintain the same position during the whole surgical operation. The patient does, however, not succeed in doing so, at least not with sufficient reliability, so that movements of the eye occur which may seriously impair the whole ablation process.
The prior art knows so-called xe2x80x9ceye-trackersxe2x80x9d; these are means which detect movements of the eye so that the laser beam used for ablation can then be controlled (caused to follow) in accordance with the eye movements. With regard to the prior art, reference is made to the following documents:
Gobbi, Pier Giorgie et al.: Automatic Eye Tracker for Excimer Laser Photorefractive Keratectomy; Supplement to Journal of Refractive Surgery, Vol. 11, May/June 1995; in addition, Lin, J. T., Ophthalmic Surgery Method Using Non-Contact Scanning Laser, U.S. Pat. No. 5,520,679, May 28, 1996; and Manns, Fabrice, et al., Optical profilometry of poly(methyl-methacrylate) surfaces after reshaping with a scanning photorefractive keratectomy (SPRK) system, periodical APPLIED OPTICS, Vol. 35, No. 19, Jul. 1, 1996.
Furthermore, reference is made to German Utility Model 298 09 759.1. In this German Utility model white light, which is emitted by light-emitting diodes, is used for the auxiliary radiation which serves to determine the eye position for xe2x80x9ceye-trackingxe2x80x9d.
DE 197 02 335 C1 describes a laser system for treating the cornea with means for causing the laser treatment beam to follow when the eye moves relative to a reference axis. An image-recording means (CCD camera) is used for this purpose and auxiliary radiation is used for illuminating the eye for the image-recording process. In accordance with a movement of the eye relative to the reference axis, a control controls a beam control unit, e.g. a galvanometric scanner. A specific dependence with respect to the pulse repetition rate of the laser radiation is suggested for the image sequence rate at which the camera takes pictures. This prior art is here assumed to be known, and used. Summarizing, it can be stated that this prior art teaches that images of the eye are recorded by means of a camera and processed in rapid succession so as to determine movements of the eye. A change in the position of the eye (pupil position) can be determined from successive images (e.g. two successive images), and the ablation laser beam can then be caused to follow in accordance with the eye movement with the aid of suitable beam control means (e.g. the above-mentioned galvanometric scanner).
It follows that at least three different radiations are differentiated in connection with PRK (in particular LASIK). Firstly, the actual laser treatment beam which causes the ablation, secondly, the so-called auxiliary radiation, i.e. the radiation which serves to illuminate the eye so as to detect the eye position e.g. by means of the camera, and, thirdly, optionally, the so-called fixation beam which is stationary and which is intended to cause the patient to fixate always the same point with the eye (the latter is only a special option).
In the prior art, halogen light, which is introduced into the observation ray path by means of a bundle of fibres, is used for illuminating the eye. Also the use of a ring lamp or the coaxial coupling in of the illumination radiation via a surgical microscope is part of the prior art. Flexible swan necks for positioning the illumination light for illuminating the foreground of the eye, especially the comea, are known as well, so that the physician can selectively adjust the light for an optimum observation of the eye as a whole. The halogen lamps and xenon lamps used in the prior art are improvable with respect to the strain on the patient as well as with respect to the illumination quality for the physician carrying out the treatment.
When, especially during spot scanning for LASIK, pictures of the iris and of the pupil are taken by means of a camera system for determining the eye position and when the centre of gravity of the pupil is then calculated (on-line), a high contrast between pupil and iris is necessary for optimum recognition of the pupil. It turned out that the auxiliary radiation is very important with respect to its angles of incidence, wavelengths, etc. for achieving good results when the eye position is being determined.
It is therefore the object of the present invention to further develop a device for the medical treatment of the eye of the type mentioned at the beginning, in such a way that reliable results are achieved when the eye position is being determined, especially when the system is used for different eyes (patients) and under different conditions.
Solutions provided for this task in accordance with the present invention are described in the claims.
The present invention is based on the finding that particularly good measurement results with regard to the eye position will be achieved when at least two infrared radiation sources are used, the radiations of these infrared radiation sources extending at an angle of 30 to 70xc2x0, preferably between 40 and 60xc2x0, to the optical axis of the laser treatment radiation. Although the optical axis of the laser treatment radiation varies slightly during scanning, it can essentially be considered to be fixed in space; in particular, it can essentially be equated with the optical axis of the eye at rest.
According to a further variant of the invention, at least three radiation sources are provided for the auxiliary radiation, the radiations of these radiation sources extending at an angle of 30 to 70xc2x0, preferably between 40 and 60xc2x0, to the optical axis of the treatment radiation. Also in this case, the use of infrared radiation sources is specially preferred, these infrared radiation sources being preferably used in the form of light-emitting diodes (LEDs), in particular LEDs having a wavelength of approx. 810 nm.
The above-mentioned radiation sources themselves can each be composed of a large number of LEDs. A xe2x80x9cradiation sourcesxe2x80x9d in the sense of this application is then a system comprising a plurality of LEDs which are firmly connected to one another physically and the composite radiation of which is directed onto the eye from substantially the same direction.
A specially preferred embodiment is so conceived that the radiation sources define a triangle below which the eye to be treated can be positioned. The above-mentioned triangle can be positioned in a horizontal plane or in a plane which is slightly inclined relative to the horizontal. The terms xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d are here used in the usual sense, i.e. it is assumed that the patient, who lies on his back, is horizontally oriented. Since electromagnetic radiation can, for the present purposes, be considered to be independent of gravitation and since also the patient can, in principle, be oriented in an arbitrary manner, the terms xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d are here used only to give a general picture; it is assumed that the patient lies in the usual way and that his head is oriented such that the eyes face upwards.
When at least three radiation sources are used for the auxiliary illuminating radiation, a preferred embodiment of the invention is so conceived that the radiations of at least two radiation sources extend at an acute angle relative to one another towards the eye to be treated. This acute angle can e.g. be in the range of from 30 to 70xc2x0, preferably in the range of from 40 to 60xc2x0. A preferred embodiment of this variant of the invention is so conceived that, when projected onto a horizontal plane, the radiation of a third radiation source extends approximately in the direction of the angle bisector of said acute angle between the two other beams.